Processes for the deposition of nickel coatings



A. R. POPLEY 3,519,473

PROCESSES FORTHE DEPOSITION OF NICKEL COATINGS July 7, 1970 Filed July 17, 1967 United States Patent ()1 ice 3,519,473 Patented July 7, 1970 U.S. Cl. 117-107.2 10 Claims ABSTRACT OF THE DISCLOSURE Surfaces are coated with nickel by exposing the surfaces to an atmosphere comprising a mixture of nickel carbonyl vapour and carbonyl sulphide vapour. Diluent inert gas may be added to the vapour mixture.

BACKGROUND OF THE INVENTION This invention relates to processes for the deposition of nickel coatings.

Processes for depositing nickel coatings onto substrate surfaces by the straightforward thermal decomposition of nickel carbonyl at elevated temperature, for example at 180 C. to 250 C. are known. Such processes rely solely on the application of heat, and consequently require heat-resisting substrates, and thereby restrict the range of materials that may be plated. In more recent years processes for the thermal decomposition of nickel carbonyl at lower temperatures, and down to and including room temperature, through the agency of a gaseous catalyst or accelerator have been evolved. For example,

U.S. Pat. 2,881,094 describes such a process for plating nickel with the assistance of H 8 gas at temperatures between 60200 C.; but it is found that nickel can only be plated out from carbonyl gas to very low coating thicknesses when using this gas as an accelerator at room temperatures and without the application of heat, since thereafter the reaction ceases. Moreover at progressively higher temperatures, with increasing dissociation of H S, inferior lating may result due to the high thermal conductivity of the hydrogen liberated by the dissociation creating favourable conditions for the gaseous reduction of the carbonyl vapour in the region close to the plating surface, thereby causing droplet condensation of nickel over said surface. The use of CS gas as an accelerator has been discussed by Pitts in the I. of Electrochemical Society, October 1965, where it is shown that although this is an efiicacious reagent for promoting the room temperature plating of nickel from nickel carbonyl, vapour concentrations of not less than 13-15% volume CS are required to achieve acceptable levels of conversion of carbonyl vapour to plated nickel and there is substantial consumption of the CS whereby the nickel plate deposited is heavily contaminated with sulphur and appreciably with carbon.

Processes for the decomposition of carbonyl vapour with the resultant evolution of CO, when achieved by thermal means at elevated temperatures, besides imposing limitations on the substrates that may be used also result in the decomposition of CO reaction product to carbon on account of said elevated temperatures, thereby contaminating the deposited nickel plate with carbon from this source. At intermediate temperatures, say 100200 C., where an accelerator gas e.g. H S is assisted by the application of less heat, carbon contamination of the deposited plate from C decomposition persists though to a lesser extent, and there may also be contamination of the plate by decomposition of the accelerator and absorption of one or more of its component elements, e.g. S

absorption from H 5. At lower temperatures down to and including room temperature, which are particularly suitable for plating plastics and other substrate surfaces which are not heat-resistant, carbon contamination from the CO environment is substantially eliminated and thermal decomposition of accelerator reagent is minimised.

It is an object of the present invention to provide a process operable at such lower temperatures which produces deposits of nickel of acceptable plating thickness at acceptable rates with little contamination.

SUMMARY OF THE INVENTION The present invention provides a process for coating surfaces with nickel wherein said surfaces are exposed to an atmosphere comprising nickel carbonyl vapour admixed with carbonyl sulphide vapour.

The process may be performed at any temperature in the range 10-120 C., and preferably within the range 15 C. C.

The nickel carbonyl-carbonyl sulphide gaseous atmosphere may comprise mixtures in the range 1.0/0.05 to 1.0/3.0 by volume, the former ratio being especially suitable for the development of thin films.

The atmosphere may also contain a diluent inert gas. Suitable gases for this purpose are argon, nitrogen, helium, carbon monoxide, carbon dioxide or hydrogen.

Diluent gases may be used with nickel carbonyl vapour in volume relationships varying from zero to 8.0/1.0.

Solid surfaces that may be satisfactorily coated with nickel by this invention include ceramics, metals, wood, cork, cardboard, plastics and glass; a particularly useful practice of the invention being a process for the room temperature coating of nickel onto plastic surfaces.

DESCRIPTION OF THE DRAWING To enable the nature of the present invention to be more readily understood, attention is directed, by way of example, to the accompanying drawing which illustrates one schematic layout of apparatus for performing the invention.

A coating chamber 1 comprises glass dome 2 standing on a base 24 and forming a vacuum-tight seal therewith by means of a flat rubber ring 25 coated with vacuum grease. Connections with ancillary apparatus for performing the plating operation are introduced through the base 24. The means for mounting an object 5 to be coated within the coating chamber is a tripod 26 standing on a plate heater 3. When an object is heated within the coating chamber it is supported on the heated surface of the plate heater: for room temperature coating, or when the object is pre-heated externally in an oven, the plate heater only acts as a support surface for the tripod. The coating chamber is evacuated by a mechanical rotary pump 8, working in conjunction with a hot trap 9 which decomposes residual vapours. Pressure within the coating chamber is monitored by a sealed-capsule vacuum gauge 10. Nickel carbonyl vapour is admitted to the chamber from a bottle 19 containing liquid carbonyl wh ch is connected to inlet line 29 and the source of carbonyl sulphide gas is a pressurized bottle 20, similarly connected, from which said gas can be admitted to the chamber indirectly via a reservoir 22, which has been precharged by the bottle to a pressure indicated by a gauge 23, or directly through a flowmeter 21.

The plate heater 3 is designed for rapid demounting from the base 24 by providing a sealing lip and O-ring 4. Heating means for the plate-heater comprises a resistor unit 27 integral with the plate supplied with electric current through an ammeter 7, the temperature measuring means being a thermocouple 6. Atmospheric pressure is restored to the coating chamber at the conclusion of a coating operation by air admittance valve 28. The fiow 3 of vapours into the plating chamber is controlled by a series of valves 12, 13, 14, 15, 16, 17 and 18; and evacuation of the chamber by the vacuum pump 8 is controlled by vacuum valve 11. Valves 12 and 13 control the fiow of nickel carbonyl vapour: valves 14, 15, 16, 17 and 18 control the flow of carbonyl sulphide, valves 16 and 17 controlling the flow of this vapour from and to the reservoir 22 respectively and needle-valve 18- controlling the flow through the flowmeter 21.

EXAMPLE OF THE PRESENT PROCESS Example 1 In one example of the use of the above-described apparatus a disc of clean polyurethane, of density 0.6 gms./ cc. and size 4 ins. dia. x 2 ins. thick was degreased with an aqueous solution of Teepol detergent, rinsed in clean water and dried in hot air before being pre-heated to a temperature of 100 C. in an oven and placed with a fiat surface uppermost on the tripod 26 constructed of PTFE material (polytetrafluoro-ethylene) standing on the plate heater 3. The tripod provided the disc with pinpoint support from each of the support legs such that the unexposed surface area of the disc was negligible: in this manner complete coverage was achieved.

The glass dome 2 was placed in position and all valves 11 to 18 were closed. The rotary pump 8 was started and the hot trap 9 was switched on to establish a hot trap temperature of 100 C. Valves 11, 13, 14, 16, 17 and needle-valve 18 were then opened and the system pumped down to a pressure of approximately 1.0 mms. of mercury, after which valves 11, 16 and 18 were closed and valve was opened for filling the reservoir 22 with carbonyl sulphide vapour to a pressure of 450 mms. of mercury from bottle 20, which vapour was then sealed off 'by closing valve 17. Valve 12 was then opened to admit nickel carbonyl vapour to the coating chamber from bottle 19 to a pressure of 50' mms. of mercury therein. Bottle 19 was then isolated by closing valve 12 and valve 16 was opened allowing carbonyl sulphide vapour at 450 mms. mercury pressure to be released from reservoir 22 into the coating chamber 1 resulting in a pressure increase of 2.0 mms. of mercury therein. Valve 16 was closed to isolate the reservoir by-pass circuit and needle valve 18 was opened to produce a flow of carbonyl sulphite vapour into the coating chamber at a rate of 5 ccs. per min., this flow rate being monitored by the flow-meter 21. The coating reaction that then ensued is thought to occur in two stages, in rapid succession, as follows:

The resultant increase in pressure due to the decomposition of nickel carbonyl vapour and the evolution of carbon monoxide may be utilised as a measure of the amount of nickel released. Due to the high density of nickel carbonyl this vapour tends to concentrate in the lower regions of the coating chamber as a coating operation progresses with the carbon monoxide gas of lower density forming a gas blanket over the heavier vapour: this causes the reaction between nickel carbonyl and carbonyl sulphide vapours to occur preferentially in the lower regions of the chamber and deposition of nickel onto surfaces in this lower region, other than the article being coated, may be retarded by covering said surfaces with a thin layer of vacuum grease. In this example, after a min. coating period and with a recorded pressure of 145 mms. of mercury in the coating chamber valves 15 and 14 were closed, valve 11 was opened and the system pumped down to clear residual coating and accelerator vapour and carbon monoxide reaction product from the coating chamber. Valve 11 was then closed and air admittance valve 28 was opened to restore atmospheric pressure within the chamber enabling glass dome 2 to be lifted off and the sample to be removed.

The polyurethane disc was coated with bright nickel, the coat being satisfactorily adherent to the disc surfaces. The thickness of the nickel coating was 123x10 ins.

The amount of nickel deposited on a solid surface and hence the thickness of the resultant nickel coating depends upon both the plating temperature and the period of time for which said surface is exposed to the coating atmosphere. In practice the solid surfaces can be exposed to the coating atmosphere for periods of time from 5 seconds up to several hours e.g. 3 hours, when very thick coats have to be deposited. The thickness of the nickel coated onto the solid surfaces can vary from 350 A. for very thin continuous films up to 0.25 in. for very thick coats, a more usual range of coating thickness being from 0.001 in. for thin coats up to 0.010 in. for thick coats. For example. with a favourable plating temperature of 50 C. very thin but continuous films of approximately 350 A. thickness may be deposited in 5 seconds. The influence of temperature on the thickness of nickel coatings is illustrated by a maximum deposition of 5500 A. at 25 C., whereas at C. there is no upper limit to coating thickness. An average thickness of nickel deposits for thin coats is approximately 0.003 in. for which a favourable plating temperature is 70 C., and an average thickness of nickel deposits for thick coats is approximately 0.007 in. for which a favourable plating temperature is 100 C.

In Example 1 carbonyl sulphide was admitted to the coating chamber 1 indirectly via reservoir 22 and directly through flowmeter 21. The carbonyl sulphide admitted via the reservoir results in a rapid pressure increase in the coating chamber from 50 to 52 mms., further carbonyl sulphide being added at a steady rate of 5 ccs. per minute through the flowmeter. The admission of carbonyl sulphide to the coating chamber indirectly via the reservoir 22 allows a sufficient concentration of carbonyl sulphide to be established in the coating chamber for reaction to commence at a reasonable rate and for deposition of nickel to ensue. Processing time is thereby saved, due to the initial rapid pressure rise, the reaction being maintained by the steady rate feed of carbonyl sulphide through the flowmeter 21.

Example 2 In a further example the afore-described apparatus was used without the use of the flowmeter 21, which was isolated by valve 18. In the further example the present process was used for the plating of closely packed bundles of filaments. A bundle of graphite filaments having a diameter of approximately 1 min. was composed of filaments each having a diameter of 6.75 microns and a length of 35.48 cms.; the bundle, consisting of 10,927 filaments held together by surface attraction, was suspended by a thread from the roof of the coating chamber, the thread being attached to one end of the bundle.

The apparatus was operated as heretofore disclosed, with an ambient temperature varying between 20.521 C. in the coating chamber during a 6 hour plating cycle and and a hot temperature of 210 C., excepting that after nickel carbonyl was admitted to the coating chamber to develop a carbonyl vapour pressure of mms. the whole of the carbonyl sulphide required for reaction was admitted to the coating chamber from the reservoir 22 to de velop a total vapor pressure of 300 mms. therein, the ratio by volume of COS to Ni(CO) being 1:1. Plating was :maintained for 6 hours, the pressure in the coating chamher having then reached 338 mms. The coating chamber was then evacuated to remove residual vapors and carbon monoxide, and atmospheric pressure restored therein.

The nickel-plated graphite filaments had an average coating thickness of 1474 A. of smooth bright nickel which did not flake off or lack adherence when the filaments were flexed.

The practice of the present invention is not limited to apparatus as herein described for the process can also be performed with apparatus wherein a stream of nickel carbonyl vapour and a stream of carbonyl sulphide are introduced separately and continuously into a coating chamber and are admixed therein; the fiow rates being adjusted, in conjunction with the fiow of efiluent gas, to maintain constant pressures within the coating chamber.

Mixtures of nickel carbonyl and carbonyl sulphide may also comprise varying proportions of diluent inert gas. In particular, in continuous flow systems, the use of a diluent gas .eg. argon or carbon monoxide enables faster linear velocities to be used and permits of operation at higher pressures. The volume ratio of diluent gas to nickel carbonyl vapour may vary from zero to about 8: 1.

In the foregoing examples the ratio of the reacting concentrations is derived from the partial pressures exerted by the reacting vapours in the constant volume coating chamber 1. Where the total concentrations of reacting vapours are admitted at the commencement of plating, as in Example 2, the ratio of nickel carbonyl vapour to carbonyl sulphide vapour by volume is derived from the ratio of the partial pressures. In Example 1, however, the ratio of the total concentrations of reacting vapours is derived from the initial 50 mms. pressure of nickel carbonyl vapour (which, for a 10.4 litre coating chamber is equivalent to 684 ccs. of carbonyl vapour at 760 mms. pressure, at an ambient temperature of 22 C.) and the initial 2 mms. pressure of carbonyl sulphide vapour plus the steady rate flow of 5 ccs. per min., maintained for the duration of the coating period of min. (equivalent to 127.4 ccs. of accelerator vapour at 760 mms. pressure, at 22 C.); the ratio of carbonyl vapour to accelerator vapour being 1.0/ 0.19 by volume: the steady rate flow of 5 ccs. per min. is maintained against the moderate rise in pressure in the coating chamber 1, from 52 mms. to 145 mms.

I claim:

1. A process for coating a surface with nickel wherein said surface is exposed to an atmosphere comprising nickel carbonyl vapour admixed with carbonyl sulphide vapour.

2. A process as claimed in claim 1 wherein the ratio of nickel carbonyl vapour to carbonyl sulphide vapour is in the range 1.0/0.05 to 1.0/ 3.0 by volume.

3. A process as claimed in claim 2 in which the temperature is from 10 C. to 120 C.

4. A process as claimed in claim 3 in which the temperature is from 15 C. to C.

5. A process as claimed in claim 1 wherein the atmosphere also comprises a diluent inert gas.

6. A process as claimed in claim 3 wherein the atmosphere also comprises a diluent inert gas, the ratio of diluent inert gas to nickel carbonyl vapour having a maximum value of 8.0/1.0 by volume.

7. A nickel-coating process according to claim 3 wherein said surface is of a plastic material.

8. A nickel-coating process according to claim 3 Wherein said surface is an outer surface of each filament of a closely-packed bundle of filaments.

9. A nickel-coating process according to claim 1 wherein said surface is an outer surface of each filament of a closely-packed bundle of graphite filaments.

10. A process as claimed in claim 8 wherein said filaments are graphite filaments.

References Cited UNITED STATES PATENTS 2,881,094 3/1959 Hoover 117107.2 3,071,637 1/1963 Horn et al.

3,097,962 7/1963 Whitacre et al 117l07.2 3,335,027 8/1967 Pitts et al. 117-107.2

OTHER REFERENCES Pitts, Jr., Journal of the Electrochemical Society, vol. 112, No. 10, pp. 1054 and 1055 relied upon.

Powell et al. Vapor Deposition, copyright May 10', 1966, pp. 305 to 307 relied upon.

ANDREW G. GOLIAN, Primary Examiner 

